A Cellulose fiber is a substance having a basic skeleton of all the plants, is accumulated on the earth in an amount exceeding one trillion tons, and is a resource that is renewable by planting trees, and therefore effective utilization thereof is desired. Although the weight is one fifth of that of steel, the cellulose fiber has strength five times stronger than steel and a low linear thermal expansion coefficient as low as 1/50 of glass fiber. Thus, a technology in which cellulose fiber is contained as a filler in a matrix of a resin or the like to impart mechanical strength to the resin is proposed (Patent Literature 1). Moreover, in order to further improve the mechanical strength of a cellulose fiber, there is a proposal on a fibrous resin reinforcing agent in which the cellulose fiber is defibrated so that a cellulose nanofiber (CNF or microfibrillated plant fiber) may be present in a dispersed state in an additive (Patent Literature 2). Furthermore, as a material obtained by subjecting a cellulose fiber to defibration treatment in the same manner as in producing the CNF, a cellulose nanocrystal (CNC) is known. The CNF is a fiber that is obtained by subjecting a cellulose fiber to defibration treatment such as mechanical defibration and that has a fiber width of about 4 to about 100 nm and a fiber length of about 5 μm or more. The CNC is a crystal that is obtained by subjecting a cellulose fiber to chemical treatment such as acid hydrolysis and that has a crystal width of about 10 to about 50 nm and a crystal length of about 500 nm. These CNF and CNC are collectively called as nanocellulose. Nanocellulose has a high specific surface area (250 to 300 m2/g) and has a lighter weight and a higher strength when compared with steel.
The thermal deformation of nanocellulose is smaller when compared with that of glass. Nanocellulose having a high strength and a low thermal expansion is a useful material as a sustainable type resource material, and creation and development of, for example, a composite material achieving a high strength and a low thermal expansion by combination of nanocellulose and a polymer material such as a resin; an aerogel material; an optically anisotropic material making use of a chiral nematic liquid crystal phase formed by self-organization of CNC, and a high-functional material obtained by introducing a functional group into nanocellulose have been made. On the other hand, nanocellulose plentifully has hydroxy groups and therefore is hydrophilic and strongly polar, which makes nanocellulose inferior in compatibility with general purpose resins that are hydrophobic and nonpolar. Therefore, in the material development using nanocellulose, studies have been conducted on improving the compatibility of nanocellulose with general purpose resins by modifying the surface of nanocellulose or introducing a functional group into nanocellulose through chemical treatment. That is to say, studies on improving dispersibility of nanocellulose to general purpose resins have been conducted.
Moreover, in the preparation of a general purpose resin composition containing a cellulose fiber as a filler, studies have been conducted on improving the dispersibility and compatibility of a cellulose fiber with general purpose resins by adding a dispersant. In Non Patent Literature 1, the dispersibility of a cellulose nanocrystal (cellulose nanowhisker) inorganic solvents is improved by adsorbing a surface active agent to the cellulose nanocrystal. In Non Patent Literature 2, an isotactic polypropylene (iPP) composite material is prepared using, as a reinforcing material, a cellulose nanocrystal to which a surface active agent is adsorbed and the tensile strength of the composite material is improved about 1.4 times stronger than that of the iPP alone. In Patent Literature 2, when cellulose is utilized as a reinforcing material for a thermoplastic resin, an additive (low-molecular weight surface active agent) having affinity to a cellulose fiber and having a particular HLB (hydrophile-lipidophile balance) value is used to create a state in which the cellulose fiber is dispersed in the additive for the purpose of suppressing the occurrence of cellulose aggregates and uniformly dispersing cellulose in a resin.
In any of the above-described conventional examples, tries to improve the dispersibility of nanocellulose by using a low-molecular weight compound as a dispersant have been made. On the other hand, the present inventors have recognized that it is extremely useful for putting a polymer dispersant into practical use that the polymer dispersant, which has been developed for dispersing a fine and hydrophobic substance such as a pigment in a resin or an aqueous medium, can be applied to cellulose being a hydrophilic substance in a simple manner and in an environmentally conscious manner that never uses a large amount of organic solvents. However, as described above, while the conventional polymer dispersants are intended to disperse a pigment or the like that is a fine and hydrophobic substance in resins or other materials, cellulose is a hydrophilic substance, is light in weight and easy to aggregate, and is hard to disperse particularly in general purpose resins, and therefore the conventional polymer dispersants cannot be applied to cellulose in the same manner as in the case of dispersing the pigment or other materials. That is to say, in order to achieve the above-described objects, the development of a polymer dispersant having a structure that can exhibit a desired functionality to cellulose that has characteristics as described above is required.
It is considered herein to have technical advantages as listed below that a polymer dispersant can be used for dispersing cellulose in general purpose resins. First of all, polymers having wide variety of structures can be designed according to monomer design, and therefore molecules can be designed according to the purposes and applications. That is to say, numerous structures as polymer dispersants can be designed and therefore synthesis of higher-performance dispersants, which are fitted to the kinds or other properties of resins to be dispersed, according to molecular design can be expected. It is considered that various kinds of polymers such as olefin-based polymers, acrylic-based polymers, ester-based polymers, and urethane-based polymers can be used as the polymer dispersant. Among the polymers, it is anticipated that acrylic-based polymers in particular are more useful because the acrylic-based monomers are polymerizable under a calm condition to give polymers in a relatively easy manner and wide variety of acrylic-based monomers exist, making molecular design according to the purposes and applications easy by selecting a target composition from among numerous compositions.
Thus, the present inventors have determined to conduct studies on making an acrylic-based polymer a polymer dispersant for cellulose. Further, it is anticipated in making an acrylic-based polymer a polymer dispersant for cellulose that a precision synthesis method is required in order to obtain an acrylic-based polymer that has a particular structure and that is useful for dispersing cellulose that plentifully has hydroxy groups and therefore is inferior in compatibility with hydrophobic general purpose resins not having polarity. Accordingly, the present inventors have considered that it is preferred to make use of a synthesis method employing living radical polymerization with which it is known that acrylic-based polymers having a particular structure can be synthesized. That is to say, the living radical polymerization method can prevent coupling and disproportionation each being a side reaction of radical polymerization, can control molecular weight, and can make a molecular weight distribution narrow by terminal radicals being stabilized. Moreover, since terminal radicals can be stabilized, addition of additional monomer to a reaction system subsequently to polymerization of a certain monomer allows polymerization to progress again and polymer segments each having a different structure can be produced, thereby making it possible to synthesize a block copolymer having a plurality of polymer segments each exhibiting a different functionality.
On the other hand, it is considered that a polymer dispersant for cellulose, the polymer dispersant having a block copolymer structure having therein a plurality of polymer segments each having a different functionality, is useful for the polymer dispersant for cellulose as an object of the present invention from the following reason. The block copolymer has a structure in which two kinds or more of polymer segments each having different components are included in one polymer chain and therefore has an advantage in that different functionalities can be imparted to respective polymer segments by devising monomer compositions. For example, to take an A-B type block copolymer consisting of a chain A and a chain B each having a different monomer composition (different monomer components) as an example for description, when the A-B block copolymer can be designed so that the polymer segment A (chain A) may have a component that has a high affinity to general purpose resins and the polymer segment B (chain B) may have a component that adsorbs to cellulose, the chain A and chain B act effectively to general purpose resins and cellulose respectively by making use of the copolymer as a dispersant for cellulose, and therefore suppression of aggregation of cellulose and stabilization of dispersion of cellulose in general purpose resins can be expected. That is to say, in the case where an A-B block copolymer having a resin-affinitive segment A and a cellulose-adsorptive segment B is used as a dispersant for a cellulose-dispersed resin composition, the dispersibility of cellulose in general purpose resins that are utilized for various kinds of shaped bodies or other products becomes favorable and sufficient enhancement of the mechanical strength of shaped bodies or other products can be expected for the A-B block copolymer as a filler.
As described previously, the living radical polymerization method is suitable for synthesizing such a block copolymer. Various kinds of methods as listed below are specifically reported as the living radical polymerization method. For example, a nitroxide method (Nitroxide Mediated Polymerization method, hereinafter abbreviated as NMP method) that makes use of dissociation and bonding of a nitroxy radical, an atom transfer radical polymerization method (Atom Transfer Radical Polymerization method, hereinafter, abbreviated as ATRP method) in which polymerization is conducted using a heavy metal such as copper, ruthenium, nickel, or iron, and a ligand that forms a complex with the heavy metal, and using a halogen compound as an initiation compound, a reversible addition fragmentation chain transfer polymerization method (Reversible Addition Fragmentation Chain Transfer Polymerization method, hereinafter abbreviated as RAFT method) in which polymerization is conducted using a dithiocarboxylic acid ester as an initiation compound, an addition polymerizable monomer, and a radical initiator, and a method in which a heavy metal compound such as organotellurium, organobismuth, organoantimony, halogenated antimony, organogermanium, or halogenated germanium is used (Degenerative Transfer method, hereinafter abbreviated as DT method), etc. have been developed, and a wide range of research and development on such living radical polymerization methods have been conducted.